Increased complexity of networks in optical transmission systems today has brought an increasing need for functionality (colorless, directionless, and contentionless [CDC] functionality) that enables any terminal to transmit an optical signal on any wavelength channel in any path regardless of the source of the optical signal. In implementing such functionality, performances (such as an optical power level, the number of wavelength channels, and a state of wavelength channels) on all the transmission paths are monitored with optical channel monitors (OCMs) so that stable transmission quality is secured in the optical transmission systems. In related optical transmission systems, different OCMs are installed in respective transmission paths for the purpose of monitoring. With an increase in the number of transmission paths to be monitored, it will be difficult to install OCMs in all the transmission paths because installation costs will increase. Technologies, therefore, have been developed in which one OCM monitors a plurality of transmission paths by sequentially switching ports using an optical switch. Related technologies are described in Japanese Laid-open Patent Publication No. 2012-105222 and Japanese Laid-open Patent Publication No. 2008-197043, for example.
When monitoring a transmission path, the OCM monitors presence or absence of signals of wavelength division multiplexing (WDM), and calculates, for example, the central wavelength and optical power level. It usually takes approximately 250 ms for the OCM to complete the calculation for all the wavelength bandwidths (channels) in use. Thus, as the number of transmission paths (ports) to be monitored increases, waiting time (monitor polling time) before the start of monitoring at each port increases. The OCM is not capable of detecting events (such as change in the number of wavelength channels, switching of transmission paths, and signal stop) immediately if such events occur during the waiting time. Consequently, it takes more time before proceeding to the next control processing, thereby degrading the transmission quality of optical signals in some cases.